(1) Field of the Invention
The present invention relates to a novel dielectric ceramic composition adapted to be used as a material of resonators and circuit boards in the microwave frequency band. More specifically, the invention relates to a dielectric ceramic composition which contains MgO, La.sub.2 O.sub.3 and TiO.sub.2 at particular composition ratios.
(2) Description of the Prior Art
In recent years, dielectric ceramics have been extensively used in microwave applications as represented by microwave integrated circuitry, wide-spread use of gun oscillators, and employment of gallium arsenide field-effect transistors for the oscillators, accompanying the trend toward practically using mobile telephones, cordless telephones, personal wireless devices, satellite broadcast receivers, etc.
The dielectric ceramics for microwave applications are chiefly used in the resonators and must satisfy the following three principal characteristics: (1) a large specific inductivity despite a decrease in the size, since the wavelength is contracted to 1/.epsilon.r.sup.1/2 in the dielectric, (2) small dielectric loss (large Q value) at high frequencies, and (3) small temperature coefficient of the resonance frequency, i.e., small or stable temperature coefficient of the specific inductivity, .epsilon.r.
Conventional examples of the dielectric ceramics of this kind include a BaO-TiO.sub.2 type material, a BaO-REO-TiO.sub.2 (where REO denotes an oxide of a rare earth oxide, the same holds hereinafter) type material, a perovoskite crystal structure in which the site of a metal element in the perovoskite crystal structure is constituted by a plurality of elements, and an MgTiO.sub.3 -CaTiO.sub.3 type material.
The BaO-TiO.sub.2 type material exhibits a specific inductivity, .epsilon.r, of as great as 38 to 40 and dielectric loss, tan .delta., of as small as 2.0.times.10.sup.-4 but makes it difficult to obtain a resonance frequency with zero temperature coefficient, Tf, on a single phase. Moreover, the specific inductivity and the temperature dependency of the specific inductivity change greatly with a change in the composition, making it difficult to decrease and stabilize the temperature coefficient, Tf, of resonance frequency while maintaining high specific inductivity and small dielectric loss.
Examples of the BaO-REO-TiO.sub.2 type material may include a BaO-Nd.sub.2 O.sub.3 -TiO.sub.2 type material or a BaO-Sm2O.sub.3 -TiO.sub.2 type material, which can exhibit a specific inductivity, .epsilon.r, of as very high as 40 to 60; These materials exhibit zero temperature coefficient, Tf, of resonance frequency, though their Q value is smaller than 2000, in other words, their dielectric loss, tan .delta., is as great as 5.0.times.10.sup.-4 or more.
The composite perovskite type material exhibits excellent dielectric characteristics as represented by Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 but uses such expensive materials as Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 in large amounts to drive up the cost of material. Furthermore, the MgTiO.sub.3 -CaTiO.sub.3 type material exhibits the Q value which is greater than 5000 and the temperature coefficient, Tf, of resonance frequency which is zero, but exhibits the specific inductivity which is as small as 16 to 25.
As described above, none of the above-mentioned materials fully satisfy the above three characteristics that are required for the dielectric material for high-frequency applications.